Here are a few of mine...

Buckets! Any size. I use them for hauling compost, holding and carrying hand tools, weeding, collecting seeds, mixing soil, soaking transplants, and a portable seat =)

Milk jugs. just as handy as a buckets, aside from the obvious use of storing and pouring various liquids =)
I cut the top off and attach a clip to the handle. Then I can clip it to a belt loop so I can have both hands free while I'm picking. They can also be cut into strips to use as plant markers.

Plastic blinds, great for plant markers.

Scissors. I use the cheep barber scissors that come with hair clipper sets. They aren't the best for cutting hair, but they work great for deadheading, collecting seeds, etc.

Also, coffee filters for lining pots to keep soil from washing out and at the same time keeping slugs from going in.

Coffee cans - for storing kitchen scraps (and used grounds/filters at work) until full - then dumping into the compost heap.

foam peanuts - as filler in the bottom of big pots.

shot glasses - for rooting cuttings in water - fit perfectly on the windowsill!

Parmesan Containers (all clear) for fermenting tomato seeds

Starbucks used coffee filters (when I pick up coffee grinds) for lining nursery pots

Starbucks coffee bean bags (sturdy silver) for storing gardening stuff, instant planter for giving away plants.
Am going to use as sandbags when we get into rainy season.

Little plastic apple sauce containers for soaking seeds (morning glory, nasturtium, etc.)

Assorted metal tuna sized and tomato sauce sized cans for rubberbands, pens, etc. If edges are rough, I bend down with pliers and duct tape the inner rim.

Inexpensive plastic silverware sorter for twist ties, plant labels, twine, etc.

I posted a while ago on using CD-Rom spindles for storing twine, wire and the clear plastic tops for storing other stuff.